The present invention relates to an arrangement for detecting light sources. The invention applies more particularly to the detection of a source which emits or reflects light radiation whose spectral characteristics are previously known.
The detection arrangement consists of an optical receiver associated with a photo-electric detector, to detect the light radiation originating from an observed field of predetermined angular extent centered on the optical sighting axis. The received radiation consists of useful radiation originating from the source, when the latter is present in the observed field, and of interference radiation which is produced by external interference sources either directly or by reflection. The apparent area of the source to be detected in the observed field is generally small, given its own dimensions and the fact of its being at a distance from the detecting arrangements. In addition, the brightness of the light generated by the source in the direction of the detection arrangement is generally low in comparison with that of the interference radiation originating from the background, i.e., from the area of observed space outside the source. The interference radiation is generally uniform and is that produced by radiation from the sun, which may be very strong.
Thus, in a given direction of sighting, interference light energy is received in the whole of the solid angle covered by the receiving optic and after detection it produces a substantially constant interference signal or noise level of high amplitude, while the useful signal is derived from the low level of light energy which is received in the solid angle defined by the apparent area of the source to be detected.
If a useful signal stronger than the interference signal can be obtained, this allows detection, but it is only possible if the light sources area of very high intensity and are the more so the farther they are away.
In order to overcome such disadvantages, it is known to increase the signal-to-noise ratio at detection by using various techniques which will be briefly reviewed below.
One solution consists in optically filtering the received radiation in a band which is as narrow as possible and matches the spectrum emitted by the source to be detected. In such arrangements the useful radiation preferably corresponds to one spectrum line in monochromatic operation or to a limited number of adjacent spectrum lines. In this way a large proportion of the interference radiation is removed by optical filtering and does not reach the detector.
Another solution which may be used conjointly with the previous one, consists in performing a physical sweep by means of transparent slots in an opaque background which are moved transversely to the optical axis to produce a temporal modulation of the detected signal.